Isotopic evidence for an intensified hydrological cycle in the Indian sector of the Southern Ocean

The hydrological cycle is expected to intensify in a warming climate. However, observational evidence of such changes in the Southern Ocean is difficult to obtain due to sparse measurements and a complex superposition of changes in precipitation, sea ice, and glacial meltwater. Here we disentangle these signals using a dataset of salinity and seawater oxygen isotope observations collected in the Indian sector of the Southern Ocean. Our results show that the atmospheric water cycle has intensified in this region between 1993 and 2021, increasing the salinity in subtropical surface waters by 0.06 ± 0.07 g kg−1 per decade, and decreasing the salinity in subpolar surface waters by -0.02 ± 0.01 g kg−1 per decade. The oxygen isotope data allow to discriminate the different freshwater processes showing that in the subpolar region, the freshening is largely driven by the increase in net precipitation (by a factor two) while the decrease in sea ice melt is largely balanced by the contribution of glacial meltwater at these latitudes. These changes extend the growing evidence for an acceleration of the hydrological cycle and a melting cryosphere that can be expected from global warming.

1. This work used a 1D framework to disentangle the mixed layer freshwater budget, one obvious caveat of this framework is to neglect the dynamic contribution associated with the changes in the circulation, specifically the meridional displacement of the ACC. Does author expect negligible net zonally/meridionally advective contribution to the mixed layer freshwater content? I imagine that the streamwise practice can to some degree mitigate the desire of quantifying the advection contribution. Nevertheless, this should be commented on somewhere in the manuscript (e.g., the Methods).
2. How does the author obtained the P-E trend (37±5 mm yr<sup>-1</sup> per decade) for subpolar sector before computing glacial meltwater and sea ice, given that both P-E and glacial meltwater are unknown in the equation?
3. The vertical distribution of meltwater is neglected here when incorporating the glacial meltwater into the matrix. Author commented on the horizontal inhomogeneity of meltwater distribution, can author also comment on the fact that there is vertical distribution of meltwater? Even though it won't change the fact that this is still the upper bound estimate of meltwater contribution. 4. More of an outlook perspective: Author made comment that these results are not able to be extrapolated to other parts of the Southern Ocean. I can also imagine that the streamwise practice will collapse in region dominated by gyre regime. Given that the reanalyses P-E are highly uncertain, does this mean that there is little can be said about the pan-Antarctica freshwater contribution from glacial meltwater, sea ice and P-E on the observational basis?
Reviewer #2 (Remarks to the Author): Akhoudas et al use observations of salinity and seawater oxygen isotope to reconstruct changes in the hydrological cycle in the Indian Sector of the Southern Ocean. Detecting changes in the hydrological cycle is challenging, especially in the Southern Ocean where observations are very sparse. The authors identify an intensification of the hydrological cycle in this region by combing unique datasets that include a long (since the 1990s) time series of seawater oxygen isotopes. The analysis is accurate and the results are very interesting and appropriate for Nature Communications. In particular, these findings highlight how seawater isotopes can provide information that other measurements can't, especially in terms of intensification of the hydrological cycle, changes in sea ice and ice sheets.
I have a main comment and some minor comments below.
• Line 229. The dataset used for sea ice freshwater fluxes covers the period 1993-2008. However, sea ice has shown major changes after 2008, in particular after 2016 with a strong decline. Could these changes impact salinity and O18 in your study region (e.g. through local or upstream changes)?
Minor Comments • Figure 1d. Does the black line show the regression south of 46°S? Please specify in the caption.
• Line 101-126: I would include the equations in the main text, or explain a bit more how P-E is estimated.
• Line 167-169. It might be worth mention that seawater oxygen isotopes show a much stronger signal of glacial meltwater near the Antarctic coast (both in East and West Antarctica), corroborating your conclusion that the impact of glacial meltwater is (at present) mostly felt in the continental shelf region.
• Line 177: I would add here a reference and a brief discussion of what shown in Morrow and Kestenare (2014; Journal of Marine Systems) as they suggest that changes in surface salinity south of Australia are not only associated with precipitation.
• Line 185: Does the depth of the measurements influence the spatial pattern and time changes? I guess not much as the data are from the mixed layer, but better to include few words about this.
• Line 238: Changes in the input of the freshwater from the Antarctic Ice Sheet is not easy to extract from observations. It occurs mostly through ice shelf melting and iceberg calving. What about combining estimate of ice shelf basal melt (e.g. Adusumilli et al., 2020; Nat. Geosci.) and estimate iceberg discharge (Greene et al., 2022; Nature)?
• The impact of ice sheet melting is small this "far north" and therefore changes in S and O18 are likely due to changes in sea ice or P-E. These contributions are linked as snow accumulates on top of sea ice. Sea ice is advected to the north and it can carry snow into the study region and then melt. This is still part of the hydrological cycle. I would mention this, potentially discussing whether the signal you are observing might capture both local and non-local contributions.
Reviewer #3 (Remarks to the Author): Review of "Isotopic evidence for an intensified hydrological cycle in 1 the Indian sector of the Southern Ocean" by Camille Hayatte Akhoudas et al.
General Comments: This paper tackles the evidence for an accelerated hydrological cycle using ocean salinity and oxygen isotopes. Speeding up of the global hydrologic cycle due to climate change is a clear outcome of the assessment of the scientific literature from IPCC. However the question of the role of the acceleration of the water cycle is quite unclear because of the confounding processes in the Antarctic (and Arctic region) from freshwater transport of sea-ice, changes in precipitation, and the potential contributions of the ice sheet melt adding to these processes.
The scientific literature is unclear on which processes are significant in the freshwater balance around Antarctica and indeed that is the virtue of this paper. The wider research community is focussed on the contributions of the Antarctic mass loss (and its freshwater signal) to ocean salinity.
This observational paper has the virtue of using oxygen isotopes and ocean salinity change (with a simple) ocean surface water balance to make the case for the varying contributions of these various freshwater processes north and south of 46S. The oxygen isotopes have different fractionated states depending on the material, meteoric ice, seaice or rain. The surprising result to me from this paper is that in the region south of 46S, where there are all three processes operating, that the subpolar waters are freshening primarily from changes in P-E, with the second largest term being the reduction in the northward transport of sea-ice (and thus melt) and the even smaller contribution from glacial melt water. So the surprise (and also confirmation of the large-scale freshening of the surface waters) is that this freshening signal is mainly driven by increased precipitation (minus evaporation). And that the two other terms are weaker by an order of magnitude and coincidentally cancel each other out. I like this result, and just emphasises the importance of taking into account precipitation, and that the changes in sea ice and glacial meltwater are still weak contributions to the water cycle changes. These results provide a consistency check on the observed (but weak) circumpolar freshening of surface salinity.
So the innovation is the use of a unique surface data set that has this capacity to differentiate the different sources of freshwater sources and affirm that there is direct observation evidence in this region.
I have some doubts about the reliability of the overall estimate (see the specific comments). The trends in Figure 3, in the sense if I removed some of the values the trends could be much reduced or change sign, seem fragile and would benefit from a generalised cross validation.
The appropriateness of the salinity balance equations. Equations 4 and 5 are highly idealised versions of the salinity balance, tailored to make use of the relatively low amount of data in this time series. Is it possible to use the mean fields from a reanalysis product (eg ECCO) and include a tracer advection of oxygen isotope as a way to verify the surface salinity changes?
Line 104: it is asserted that influence on sea-ice is limited to the southern region, but there is no evidence to support. The westerly winds and Ekman should carry these surface waters further northwards. Maybe a Mazloff et al paper, on ECCO could cited as evidence, if appropriate. This is a key point for the argument that follows and the separation of the analysis of surface salinity.
Line 106 to 112: It was not clear to me how equations 4 and 5 are actually solved (and this comment includes my reading of the supplementary material). How was the uncertainty of the oxygen isotopes taken into account. Did the bootstrapping including working across the variation in the oxygen isotopes values (eg glacial melt -24+-3 per mill.) Line 130 to 132: The entire discussion here is about the surface salinity value. A salinity budget should of course include at least the thickness of the surface mixed layer, and if this had thinned at later times, then there would be even more fresh values. Has this thickness been taken into account or assessed explicitly?
Line 141-143: The error bars are carefully given for the rates of change per decade (either for oxygen isotopes or surface salinity). The error bars are often relative larger for the northern region compared with the southern region, and different for the three processes. Can some insight be given to variations in the relative accuracy for surface salinity and oxygen isotopes be given?
Line 149-151: Equations 4 and 5 depend on the mean isotopic values and the the error ranges, are these ranges taken into account in the boot-strapping solutions in surface salinity error bars? Line 159-160: ".. leading to a net surface ocean freshening… ", I think it needs to be made clear that this ocean freshening is for the region where the sea-ice is transported too, rather than say the coastal or polynya regions where the sea-ice is formed "surface salinity" would increase from this contribution.
Line 167: "... only a limited influence of an increase in Antarctic discharge on the surface water …" Line 176-176: This last sentence is at odds with the earlier assertion that the larger scale surface ocean salinity map and the changes described in this region is consistent, and here they should not be extrapolated to the whole southern ocean. And inconsistent with the discussion in the methods section. I think is is useful to provide the back of envelope estimates for the whole of Southern from this sector, keeping in mind these are trends that are derived from data nearly 30 years.
Couple of specific comments: the paper reports the average surface salinity changes. But actually what is changing is the depth integrated mixed layer salt content, and P-E is reported as g/kg per decade. So it would be helpful to have a table of the more common flux changes, so P-E would be mm/year per decade. This would allow a comparison with the NCEP or ERA P-E values. In this work I have not seen a table or the equivalent P-E values from NCEP (I note the discussion in methods) compared with the results from this work. The inhomogenieitiess and inconsistencies become a case that support these measurements. Such a table would combine the re-analyses products, the ice melt and the sea-ice transport values altogether.
There is no discussion of the trends of temperature in the measurements. Has the mixed layer thinned and so these freshening values in the southern ocean are a result of a thinner mixed layer. Its unclear whether the mixed layer depth is variable in space (in the salinity balance equations).  . Looking in these error bars I wondered how robust the trends are in the trends, and if one was to remove a few values from each panel, whether the estimates of the trends would be as stable as suggested by the linear trends with associated errors. There are relatively few degrees of freedom, and looking at the seasonal values, there is some serial correlation. It would be good to test the reliability of the trends here by subsetting the values to see if the estimates are table, and then say so.
I liked the additional materials in the supplementary materials on the estimate of sea-ice melt and also the comparison of the three reanalysis products, and their similarities and differences. While these results are not exactly what I would have anticipated they make the case that the oxygen isotopes do have a lot to offer in terms of making estimates of the strength of the P-E to the north of 46S and the region to the south of 46S.
The additional text around the methods and the solution of the 1-D freshwater balances and the apriori estimate of freshwater from sea-ice is much clearer now. The response to my comments are satisfactory except where I note below. I have some additional comments below about strengthening the message of this paper (in abstract and in the main text).
The recognition of the mixed layer, and better discussion of the salinity balance.
The figures are clear, and the written text is of high quality. The supplementary materials are complete and appropriate.
I believe the manuscript is acceptable for publication.
Comments below on this revision.

Abstract:
Line 18, middle sentence, ".. decreasing it in subpolar surface waters …' reads better I think "...decreasing salinity in subpolar surface waters by …" At first I thought it referred to the atmospheric water cycle.
There is a common misconception that all of the salinity changes are caused by melt from Antarctica, so I suggest that the last sentence, could be augmented by inserting the following sentence before the last sentence with some like "The oxygen isotope data show that the freshening in in subpolar waters is largely driven by the increase in P-E (by a factor of two) while the decrease in sea-ice melt is largely balanced by the contribution glacial melt water at these latitudes. These changes… " The message here could be equally well accomplished by re-phrasing the text in the current second last sentence. To me the power and success of this paper is the use of the oxygen isotopes and the power they give to discriminate the different freshwater processes.
The text throughout has error bars, but not in the abstract, perhaps add.
Maine text: Specific comments. Line 64: To be clear "… local anomaly .." should be a "... local spatial anomaly…" Line 131 to 133. Suggest "...per decade, which is a relative change that would be more than …" to "... per decade, and this deepening represents a third of the relative change in P-E based on the atmospheric reanalysis estimates (ERA5 …" Line 134 to 154: In all of these estimates the seasonal cycle is not mentioned, and yet later in the text under methods the seasonal cycle is discussed in the integration of the freshwater fluxes (line 226). It would be great to include an explicit statement that the seasonal cycle has been taken into account by integrating over the season or by saying that the estimates presented are the annual averages (thus taking into account the seasonal cycle).
Line 181: The sea-ice decline is really a statement of less sea-ice melt in this zone compared with earlier times, and since extent has not changed it really means that the amount ice formed nearer to the continent has actually reduced and the ice has thinned (overall), ie volume of sea-ice is declining. I suggest bring this point out more firmly. "... sea ice decline (implying less formation and volume) induces …" Line 209 to 215: In the first revision I noted a "logical inconsistency between the acceleration of the hydrological cycle, which is more a global parameter, revealed more through zonal averages. So perhaps not drawing to negative argument, that the results are only applicable to this region, and not elsewhere -which raises larger questions about spatial and temporal variations and uncertainty in that context, it would be better extol the virtues of the oxygen isotopes to pull apart the various processes and thus provide the first quantifiable evidence of the strength of the different freshwater processes in this region, and then extend to the larger region. I would reframe this way. At the moment it undermines the great results you have collectively for this region.